1. Field of the Invention
The present invention relates to a base station, a mobile communication system, and a communication method.
2. Description of the Related Art
Conventionally, a mobile communication system adopts a cellular system, which covers service areas two-dimensionally by arranging a plurality of base stations to form a plurality of cells. In addition, the mobile communication system adopts a sector-cell structure that covers the service area by dividing a cell into a plurality of sectors and placing antennas for each sector on the base stations. In the mobile communication system, directional beam reception/transmission techniques using adaptive antenna array diversity are adopted, in order to suppress the interference power from other mobile stations. Adaptive antenna array diversity is a technique for adaptive reception of a base station in an uplink and adaptive transmission of a base station in a downlink.
In the uplink, adaptive antenna array receive diversity is performed. This is a technique where a base station performs directional beam reception by receiving data from mobile stations with a plurality of antennas, multiplying appropriate weights to the signals received by respective antennas, and combining them. For example, a coherent adaptive antenna array diversity reception method using pilot symbols in a radio access referred to as direct sequence code division multiple access (DS-CDMA) has been proposed (for example, ‘Pilot Symbol-Assisted Decision-Directed Coherent Adaptive Array Diversity For DS-CDMA Mobile Radio Reverse Link’, S. Tanaka, M. Sawahashi, and F. Adachi; IEICE Trans. Fundamentals, vol. E80-A, pp. 2445–2454, December 1997).
In addition, in the downlink, adaptive antenna array transmit diversity is performed. For example, a method has been proposed as the adaptive antenna array transmit diversity where a base station performs directional beam transmission after performing compensation for amplitude/phase variance occurring in a radio link of a beam pattern formed in the uplink (for example, ‘Adaptive Antenna Array Transmit Diversity In FDD Forward Link For WSDMA And Broadcast Packet Wireless Access’, H. Taoka, S. Tanaka, T. Ihara, and M. Sawahashi; IEEE Wireless Communications, pp. 2–10, April 2002).
The mobile communication system reduces the interference power from mobile stations existing in the same sector, another sector, or another cell by adopting such adaptive antenna array diversity. FIG. 1 is a diagram illustrating a configuration of a base station 310, which performs directional beam reception/transmission that adopts such adaptive antenna array diversity. FIG. 1 shows a case where a cell is divided into three sectors: a first sector, a second sector, and a third sector. The base station 310 has a first sector array antenna 311a, a second sector array antenna 311b, and a third sector array antenna 311c for transmitting a directional beam to respective sectors.
The base station 310 then transmits a downlink channel to mobile stations by using the first sector array antenna 311a, the second sector array antenna 311b, and the third sector array antenna 311c, respectively. The downlink channel includes a dedicated channel, which transfers individual data such as individual information data of each mobile station, and a common channel, which transfers common data such as control data common to a plurality of mobile stations.
As shown in FIG. 2A, the base station 310 transmits the dedicated channel with a directional beam, by using the array antenna 311. For example, the base station 310 transmits abeam to a mobile station 320a of a user #1 by narrowing the beam so as to form a transmission beam pattern 304a for the user No. 1. Similarly, the base station 310 transmits a beam to a mobile station 320b of a user #2 by narrowing the beam so as to form a transmission beam pattern 304b for the user #2. As a result, the interference power between the mobile stations 320a and 320b may be decreased.
Meanwhile, in the case of transmitting a common channel, the base station 310 transmits the common channel with an omnidirectional beam, by using the array antenna 311 so as to form a common channel omnidirectional beam pattern 305 as shown in FIG. 2B. This aims to enable all mobile stations 320a and 320b in the sector to receive the common channel. There are two conventional methods for transmitting a downlink common channel with the omnidirectional beam, by using the array antenna 311 so as to cover the entire sector. A first conventional method is a method where an antenna weight for a plurality of antenna elements configuring the array antenna 311 is determined so as to form an omnidirectional beam pattern that covers the entire sector. A second conventional method is a method where omnidirectional beam transmission is performed using only one of the antenna elements configuring the array antenna 311.
In addition, a method has been proposed where a base station has an omnidirectional antenna and a directional antenna, transmits common information such as a control signal using the omnidirectional antenna, and transmits the directional beam for subsequent communications, by using the directional antenna (Japanese Patent Laid-Open No. Hei 11-289287, and Japanese Patent Laid-Open No. Hei 10-173585).
However, with the first conventional method, beam gain varies depending on the angle of beam transmission, and a drop in beam gain occurs. Therefore, saturation power of a transmission amplifier must be increased to compensate for the drop in beam gain. In addition, with the second conventional method, antenna gain becomes 1/the total number of the antenna elements configuring the array antenna 311, because only one antenna element is used for transmission. More specifically, since the array antenna is configured by arranging a plurality of the antenna elements, the length of antenna elements is shorter than that of the sector antenna. This is because there is a restriction in wind pressure given to an antenna in proportion to the cross sectional area of the antenna, and the length of antenna elements must be shortened as the number of antenna elements increases. Since the base station transmits the omnidirectional beam, by using only one of such short antenna elements, antenna gain is further decreased. Therefore, the saturation power of the transmission amplifier must be increased in order to cover the entire sector. As a result, the equipment scale of the base station has increased in both cases of the first conventional method and the second conventional method.
In addition, mobile stations include those capable and those incapable of receiving a directional beam. Therefore, there is the following problem even with the method where a base station having an omnidirectional antenna and a directional antenna transmits common information such as a control signal using the omnidirectional antenna, and transmits the directional beam for subsequent communications using the directional antenna. In the case of transmitting a dedicated channel, the base station transmits the dedicated channel with the directional beam using a directional antenna regardless of the type of mobile station. Therefore, the base station transmits the directional beam using a directional antenna for even the mobile station that is incapable of receiving a directional beam. Accordingly, the mobile station incapable of receiving a directional beam cannot receive signals appropriately.